Last night’s M=3.9 earthquake east of San Jose occurred near Alum Rock Park. This quake struck along the Calaveras Fault, which has the second highest probability of rupturing in a large magnitude earthquake by 2043, according to the USGS. (Photo from: SFGate)

Just after 10:30 pm last night, a M=3.9 earthquake struck east of San Jose. While this earthquake only registered weak shaking near the epicenter, according to the USGS ShakeMap, over 8,500 people filled out felt reports on the USGS website. Last night’s quake occurred at a depth of 9 km near Alum Rock Park and Mt. Hamilton.

This Temblor map shows the location of yesterday’s M=3.9 earthquake east of San Jose, as well as the three major faults around the Bay Area. Also labeled on this map is the location of Anderson Dam, which sits within a couple kilometers of the fault. If a large magnitude earthquake were to occur along the Calaveras, the dam could fail, which could flood San Jose.

Based on the USGS focal mechanism, we know that this earthquake was pure right-lateral strike-slip in nature. Given this, and the quake’s location, it struck along the Calaveras Fault, which extends from just south of Walnut Creek, to south of Hollister where it joins with the San Andreas. Historically, the Calaveras Fault has produced much larger magnitude earthquakes, including in 1984, when a M=6.2 earthquake struck near Morgan Hill, resulting in over $7.5 million in damage.

A major reason why activity along the Calaveras Fault deserves close attention is because the USGS gives it the second highest probability of rupturing in a large magnitude (M≥6.7) by 2043. The probability, 26%, trails only the nearby Hayward Fault, which has a 33% chance of rupturing. The Hayward Fault also comes into play because in 2015, UC Berkeley seismologists showed that the Hayward Fault is a branch of the Calaveras, meaning both faults could rupture simultaneously in large magnitude quakes.

While a large magnitude Calaveras Fault earthquake would generate significant shaking and residential damage throughout much of the Bay Area, a large piece of infrastructure is also at risk. Anderson Dam, a dam which has been identified as being seismically at risk, sits right next to the Calaveras Fault. While a plan to seismically retrofit the dam officially began in 2012, the earliest construction would start is 2020. If a large earthquake were to cause the dam to fail, the resulting flooding could inundate much of San Jose. Therefore, highlighting an earthquake, even a small one like yesterday’s M=3.9, is important because of the risk the Calaveras Fault poses to several million people.